Mismatched PCM and/or ABS-TC Module
Your engine control computer and anti-lock brake system module are not communicating or are incompatible versions, like trying to connect two phones with different operating systems. This prevents them from working together properly to control engine and brake functions.
The PCM and ABS-TC module perform a handshake communication check during initialization to verify software compatibility and proper CAN bus connectivity. If version mismatch is detected or communication timeouts occur, a fault is logged. The modules compare calibration IDs and firmware revisions to ensure synchronized operation.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Module Communication Handshake | Successful sync on startup; matching firmware versions | Handshake failure or version mismatch detected |
| CAN Bus Message Response | Response received within 100ms | No response or timeout after multiple attempts |
Code C1805 is classified as a serious fault. If your check engine light is flashing — not just steady — pull over safely and do not continue driving. A flashing CEL indicates an active misfire or critical failure that can cause catalytic converter damage within minutes or permanent engine harm within miles. Contact a certified mechanic immediately. Do not attempt roadside repairs on high-severity codes unless you are trained to do so.
Once the fault is repaired, C1805 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.